What kind of food are you looking for? Are you looking for a restaurant that you sit down at and eat a real meal (e.g. Canton Delights) or are you looking for the quick snack places? The dessert places? Congee places?

I know people from HK who loves Shanghainese food (lots of Shanghainese refugees who fleed the communist take over), sushi and other japanese cuisines, Taiwanese, Thai, Vietnamese, French, Italian, etc.

The noodle/jook side of Joy Luck Place is -very- good. Recent visitors from Hong Kong commented that the won ton min was better than many of the HK restaurants. If the OP's visitors want a simple and familiar meal, this is the place.

I felt that the main dining room started to go down hill a few years ago. If you're looking for dim sum or a 9 course dinner, I'd recommend Mayflower over Joy Luck Place.

Stay with Kaygetsu. The cuisine is light and there are plenty of seafood options if you think they'd be more seafood inclined because they're from HK. You must finish with the tai chazuke. The hamachi kama is also delicious if it's available.

I completely agree with Porthos on a firm Kaygetsu recommendation even though I've never eaten dinner there (but have had lunch at their sushi bar). A proper upscale kaiseki meal if available in Hong Kong would easily cost a whole lot more, and is not terribly common fare over there for the average HKer.

I would not bother with any of the Cantonese or Shanghainese restaurants in the area unless they absolutely must have Chinese.

If not Kaygetsu, Kitsho in Cupertino would be a great 2nd choice for Japanese. If they are not into raw fish, Howard has tons of great cooked dishes and I would definitely go for any tofu dish since Howard makes them from scratch (cold tofu 3 bean appetizer is awesome, and ditto for steamed tofu appetizer). There are also some white board specials such as grilled fish (not the teriyaki or hamachi kama common kind) that might be interesting. Ask for the Japanese appetizers menu (and help to translate, I'm sure there are some gems there). House made miso, using miso marination for fish, is great. Howard made a great gindara nitsuke (soy sauce steamed black cod) that was most excellent.

Tanto (Sunnyvale) or Gochi (Cupertino, northern end) might be other nice places to take them.

I don't know of great Italian or French restaurants around Cupertino at the moment, but there has to be something interesting or worthwhile that's within driving distance.

(Side note, when my uncle and aunt were visiting from Hong Kong, we took them to Il Fornaio in Burlingame, and they loved it - and they are very picky eaters).

I am hesitant on taking them to Gochi, as I have yet to try it myself. Hectic schedule these days, however being as the owner/chef is from Tanto, and it's serioulsy packed nightly I am willing to give it a go. Any suggestions?

All the other Japanese restaurants you mentioned are all on my top eats, being based out of Cupertino.

As for italian restaurants I am partial to Il Postale in Sunnyvale, the rock shrimp capelinni is fantastic and if you like chocolate they sometimes have a dessert called 'boca negra' a truely decadent slice of flourless chocolate cake. Another I like is pizza antica in santana row, their gnocchi is interesting, i like it, and the simple spaghetti w/fennel sasuage makes for a simple dinner.

I'm not a huge fan of french food, but I hear there's a restaurant or two off Big Basin in Saratoga.

You can do a board search for Gochi, and will likely find Melanie Wong's review on it with photos. I've only been for lunch, while quality is great, it is not representative of what they really can do during dinner I do believe. The minor setback is that you probably need to take off your shoes to dine, simliar to the central dining area of Tanto Sunnyvale.

It sounds like you are hitting the right spots/potential suggestions of your own. Santanna Row would be a nice little walk for out of towners to marvel (even for window shopping), then you can take them to Valley Fair afterwards. Or even take a little nice walk around downtown Los Altos or head south into Los Gatos (Manresa might be a possibility for something very high end western). Saratoga/Big Basin sounds like another nice area to hit, but I don't know of the restaurants there.

When my inlaws were here we drove even further and went to places like Sausalito, ate at Farallon (SF) and when we had to have Chinese we did Lucky Buddha in San Carlos (off the white board menu), Silver House in San Mateo, and Joy in Foster City (Chinese only menu and pre-ordered the $30 specials like stuffed duck [ba bao ya] or beggars chicken).

For the really really picky HK eaters visiting SF wanting Cantonese (just from my perspective and family/experiences), I personally wouldn't settle for anything less than say Koi Palace in Daly City if you're on a splurge, Silver House in San Mateo, R&G Lounge in SF Chinatown (I'm sure there are arguments for and against this place but it has always been good for us with few misses). Tai Wu in Daly City is more out of the way, but with its growing popularity with the famous executive chef, if you can get reservations (heard they may not take them anymore but not 100% sure) and can go on a weeknight, it might be a winner.

While Joy has great food, the hot summer weather combined with the restaurant's poor ventilation resulting in stuffiness(despite the one or two wall fans) would make the dining experience rather unpleasant. I'd hold off until the weather is cooler.